In order to be considered suitable to replace conventional film projectors, digital projection systems, particularly multicolor cinematic projection systems, must meet demanding requirements for image quality and performance. Among other features, this means high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 1,000:1.
The most promising solutions for multicolor digital cinema projection employ, as image forming devices, one of two basic types of spatial light modulators (SLMs). The first type of spatial light modulator is the Digital Light Processor (DLP) a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DLP devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DLPs are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat. No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951(Anderson); and U.S. Pat. No. 6,089,717 (Iwai). DLPs have been successfully employed in digital projection systems.
FIG. 1 shows a simplified block diagram of a projector apparatus 10 that uses DLP spatial light modulators. A light source 12 provides polychromatic light into a prism assembly 14, such as a Philips prism, for example. Prism assembly 14 splits the polychromatic light into red, green, and blue component bands and directs each band to the corresponding spatial light modulator 20r, 20g, or 20b. Prism assembly 14 then recombines the modulated light from each SLM 20r, 20g, and 20b and provides this light to a projection lens 30 for projection onto a display screen or other suitable surface.
Although DLP-based projectors demonstrate the capability to provide the necessary light throughput, contrast ratio, and color gamut for most projection applications from desktop to large cinema, there are inherent resolution limitations, with current devices providing only 2148×1080 pixels. In addition, high component and system costs have limited the suitability of DLP designs for higher-quality digital cinema projection. Moreover, the cost, size, weight, and complexity of the Philips or other suitable prisms as well as the fast projection lens with a long working distance required for brightness are inherent constraints with negative impact on acceptability and usability of these devices.
A second type of spatial light modulator used for digital projection is the LCD (Liquid Crystal Device). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size, favorable device yields and the ability to fabricate higher resolution devices, for example 4096×2160 resolution devices by Sony and JVC Corporations. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,121 (Maki et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.). LCOS (Liquid Crystal On Silicon) devices are thought to be particularly promising for large-scale image projection.
However, LCD components have difficulty maintaining the high quality demands of digital cinema, particularly with regard to color and contrast, as the high thermal load of high brightness projection affects the materials polarization qualities.
A continuing problem with illumination efficiency relates to etendue or, similarly, the Lagrange invariant. As is well known in the optical arts, etendue relates to the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image. Numerically, etendue is proportional to the product of two characteristics, namely the image area and the numerical aperture. In terms of the simplified optical system represented in FIG. 2 having light source 12, optics 18, and a spatial light modulator 20, etendue is a factor of the area of the light source A1 and its output angle θ1 and is equal to the area of the modulator A2 and its acceptance angle θ2. For increased brightness, it is desirable to provide as much light as possible from the area of light source 12. As a general principle, the optical design is advantaged when the etendue at the light source is most closely matched by the etendue at the modulator.
Increasing the numerical aperture, for example, increases etendue so that the optical system captures more light. Similarly, increasing the source image size, so that light originates over a larger area, increases etendue. In order to utilize an increased etendue on the illumination side, the etendue must be greater than or equal to that of the illumination source. Typically, however, the larger the image, the more costly and sizeable the optics and support components. This is especially true of devices such as LCOS and DLP components, where the silicon substrate and defect potential increase with size. As a general rule, increased etendue results in a more complex and costly optical design. Using an approach such as that outlined in U.S. Pat. No. 5,907,437 (Sprotbery et al.) for example, lens components in the optical system must be designed for large etendue. The source image area for the light that must be converged through system optics is the sum of the combined areas of the spatial light modulators in red, green, and blue light paths; notably, this is three times the area of the final multicolor image formed. That is, for the configuration disclosed in U.S. Pat. No. 5,907,437, optical components handle a sizable image area, therefore a high etendue, since red, green, and blue color paths are separate and must be optically converged. Moreover, although a configuration such as that disclosed in U.S. Pat. No. 5,907,437 handles light from three times the area of the final multicolor image formed, this configuration does not afford any benefit of increased brightness, since each color path contains only one-third of the total light level.
Efficiency improves when the etendue of the light source is well-matched to the etendue of the spatial light modulator. Poorly matched etendue means that the optical system is either light-starved, unable to provide sufficient light to the spatial light modulators, or inefficient, effectively discarding a substantial portion of the light that is generated for modulation.
The goal of providing sufficient brightness for digital cinema applications at an acceptable system cost has thus far proved elusive to designers of both LCD and DLP systems. LCD-based systems have been compromised by the requirement for polarized light, reducing efficiency and increasing etendue, even where polarization recovery techniques are used. DLP device designs, not requiring polarized light, have proven to be somewhat more efficient, but still require expensive, short lived lamps and costly optical engines, making them too expensive to compete against conventional cinema projection equipment.
In order to compete with conventional high-end film-based projection systems and provide what has been termed electronic or digital cinema, digital projectors must be capable of achieving comparable cinema brightness levels to this earlier equipment. As some idea of scale, the typical theatre requires on the order of 10,000 lumens projected onto screen sizes on the order of 40 feet in diagonal. The range of screens requires anywhere from 5,000 lumens to upwards of 40,000 lumens. In addition to this demanding brightness requirement, these projectors must also deliver high resolution (2048×1080 pixels) and provide around 2000:1 contrast and a wide color gamut.
Some digital cinema projector designs have proved to be capable of this level of performance. However, high equipment and operational costs have been obstacles. Projection apparatus that meet these requirements typically cost in excess of $50,000 each and utilize high wattage Xenon arc lamps that need replacement at intervals between 500-2000 hours, with typical replacement cost often exceeding $1000. The large etendue of the Xenon lamp has considerable impact on cost and complexity, since it necessitates relatively fast optics to collect and project light from these sources.
One drawback common to both DLP and LCOS LCD spatial light modulators (SLM) has been their limited ability to use solid-state light sources, particularly laser sources. Although they are advantaged over other types of light sources with regard to relative spectral purity and potentially high brightness levels, solid-state light sources require different approaches in order to use these advantages effectively. Conventional methods and devices for conditioning, redirecting, and combining light from color sources, used with earlier digital projector designs, can constrain how well laser array light sources are used.
Solid-state lasers promise improvements in etendue, longevity, and overall spectral and brightness stability but, until recently, have not been able to deliver visible light at sufficient levels and within the cost needed to fit the requirements for digital cinema. In a more recent development, VCSEL laser arrays have been commercialized and show some promise as potential light sources. However, the combined light from as many as 9 individual arrays is needed in order to provide the necessary brightness for each color.
Examples of projection apparatus using laser arrays include the following:
U.S. Pat. No. 5,704,700 entitled “Laser Illuminated Image Projection System and Method of Using Same” to Kappel et al. describes the use of a microlaser array for projector illumination.
Commonly assigned U.S. Pat. No. 6,950,454 to Kruschwitz et al. entitled “Electronic Imaging System Using Organic Laser Array Illuminating an Area Light Valve” describes the use of organic lasers for providing laser illumination to a spatial light modulator.
U.S. Patent Publication No. 2006/0023173 entitled “Projection Display Apparatus, System, and Method” to Mooradian et al. describes the use of arrays of extended cavity surface-emitting semiconductor lasers for illumination; U.S. Pat. No. 7,052,145 entitled “Displays Using Solid-State Light Sources” to Glenn describes different display embodiments that employ arrays of microlasers for projector illumination.
U.S. Pat. No. 6,240,116 entitled Laser Diode Array Assemblies With Optimized Brightness Conservation“to Lang et al. discusses the packaging of conventional laser bar- and edge-emitting diodes with high cooling efficiency and describes using lenses combined with reflectors to reduce the divergence-size product (etendue) of a 2 dimensional array by eliminating or reducing the spacing between collimated beams.
There are difficulties with each of these types of solutions. Kappel '700 teaches the use of a monolithic array of coherent lasers for use as the light source in image projection, whereby the number of lasers is selected to match the power requirements of the lumen output of the projector. In a high lumen projector, however, this approach presents a number of difficulties. Manufacturing yields drop as the number of devices increases and heat problems can be significant with larger scale arrays. Coherence can also create problems for monolithic designs. Coherence of the laser sources typically causes artifacts such as optical interference and speckle. It is, therefore, preferable to use an array of lasers where coherence, spatial and temporal coherence is weak or broken. While a spectral coherence is desired from the standpoint of improved color gamut, a small amount of broadening of the spectrum is also desirable for removing the sensitivity to interference and speckle and also lessens the effects of color shift of a single spectral source. This shift could occur, for example, in a three color projection system that has separate red, green and blue laser sources. If all lasers in the single color arrays are tied together and of a narrow wavelength and a shift occurs in the operating wavelength, the white point and color of the entire projector may fall out of specification. On the other hand, where the array is averaged with small variations in the wavelengths, the sensitivity to single color shifts in the overall output is greatly reduced. While components may be added to the system to help break this coherence as discussed by Kappel, it is preferred from a cost and simplicity standpoint to utilize slightly varying devices from differing manufactured lots to form a substantially incoherent laser source. Additionally reducing the spatial and temporal coherence at the source is preferred, as most means of reducing this incoherence beyond the source utilizes components such as diffusers, which increase the effective extent of the source (etendue), cause additional light loss, and add expense to the system. Maintaining the small etendue of the lasers enable a simplification of the optical train, which is highly desired.
Laser arrays of particular interest for projection applications are various types of VCSEL (Vertical Cavity Surface-Emitting Laser) arrays, including VECSEL (Vertical Extended Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity Surface-Emitting Laser) devices from Novalux, Sunnyvale, Calif. However, conventional solutions using these devices are prone to a number of problems. One limitation relates to device yields. Due largely to heat and packaging problems for critical components, the commercialized VECSEL array is extended in length, but limited in height; typically, a VECSEL array has only two rows of emitting components. The use of more than two rows tends to dramatically increase yield difficulties. This practical limitation would make it difficult to provide a VECSEL illumination system for projection apparatus as described in U.S. Pat. No. 7,052,145, for example. Brightness would be constrained when using the projection solutions proposed in U.S Patent Publication No. 2006/0023173. Although Kruschwitz et al., U.S. Pat. No. 6,950,454 and others describe the use of laser arrays using organic VCSELs, these organic lasers have not yet been successfully commercialized. In addition to these problems, conventional VECSEL designs are prone to difficulties with power connection and heat sinking. These lasers are of high power; for example, a single row laser device, frequency doubled into a two row device from Novalux produces over 3W of usable light. Thus, there can be significant current requirements and heat load from the unused current. Lifetime and beam quality is highly dependent upon stable temperature maintenance.
Coupling of the laser sources to the projection system presents other difficulties that are not adequately addressed using conventional approaches. For example, using Novalux NECSEL lasers, approximately nine 2 row by 24 laser arrays are required for each color in order to approximate the 10,000 lumen requirement of most theatres. It is desirable to mount these sources separately in order to provide sufficient heat dissipation as well as for running power and control signals and allowing modular design that simplifies servicing and replacement. At the same time, however, it is necessary to combine the laser beams from multiple sources in order to form a single beam that provides collimated light. Solutions that overlay individual beams lose some of the generated light due to inefficiencies in beam-combining coatings. Any angular component introduced in the combining process increases the etendue and is generally undesirable. Redirecting multiple beams with minimal spacing between beams is desirable, but not easily achieved using conventional beam-combining techniques.
Thus, it can be seen that there is a need for illumination solutions that capitalize on the advantages of solid-state array light sources and allow effective use of solid-state illumination components with DLP and LCOS modulators.